1. Field of the Invention
The present invention relates to a packaging sheet for perishable goods and more specifically to a packaging sheet which has a capability of maintaining the freshness of perishable goods.
2. Prior Art Description
A variety of methods have been employed for keeping perishable goods in a preferable condition in terms of their freshness etc. during transportation or storage. In a typical method, based on the recognition that fresh vegetables release additional-maturation hormones such as ethylene and removal of such hormones gives a prolonged freshness to the vegetables stored such as in a refrigerator, materials with a high adsorptivity to such hormones are used. For example, activated carbon particles or a sheet formed thereform are disposed in a bag in which fresh food is contained so that additional-maturation substances can be removed from the inside of the bag. Another example is show, for instance, in Japanese Patent Public Disclosure No. 61-93835 wherein porous materials having fine pores are subjected to an oxidation process to thereby become adsorptive materials with an adsorptivity to molecules of low number and oxidation-catalytic action.
The known adsorptive materials, however, cannot be expected to have a sufficient freshness-keeping ability and are expensive to manufacture, so they are not always satisfactory in practice. In addition, there is a need to apply conventional adsorptive materials with some additional process so that they are provided with some other functions such as a sterilizing effect as well as adsorptivity as disclosed in the above publication. This also increases the manufacturing cost of the adsorptive materials and as such goes against the demand for decreased manufacturing cost of these adsorptive materials.